The present invention relates to a multi-dimensional positioning apparatus having movable frames which are guided by linear guide tracks and, more particularly, to a positioning apparatus suitable for use in processes in which elimination of even small influence of thermal distorsion due to ambient temperature change is absolutely necessary, e.g., an automated assembly process for the assembly of electronic devices or apparatuses.
Positioning apparatuses of the above described type are designed in such a manner so as to exclude the use of materials having different values of thermal expansion coefficient, in order to avoid unfavorable influence of thermal distortion which may be caused by any change in the temperature, and to thereby ensure a high positioning accuracy. More specifically, major parts of the apparatus such as guide tracks and a movable frame are made of materials having similar values of thermal expansion coefficient, for instance iron and steel, and the guide tracks and other parts are directly fixed to the associated frames by bolts which also are made of a material such as steel or iron.
When the movable frame and bolts are made of material or materials similar to that of the guide tracks, however, the total weight of the apparatus is inconveniently increased. The increase in the weight of the movable parts seriously affects the positioning performance of the apparatus and often impairs the positioning accuracy. In addition, the driving system of the apparatus has to have a large power or capacity.
For eliminating the unfavorable effect of thermal distortion in a positioning apparatus, a method has been proposed in, for example, Japanese Patent Laid-Open No. 119351/1981, wherein compensation for any error in the positioning of a movable carriage is effected through the detection of the amount of change in the size of a ball screw shaft due to a temperature change, as well as the amount of deflection of the ball screw due to the weight of the movable carriage.
This proposed compensation method, however, requires a highly correct operation of the ball screw. Therefore, it is impossible to attain a high positioning accuracy, however correctly the error compensation may be conducted, if the ball screw fails to operate correctly due to a thermal distortion which may be caused by a difference in the value of friction coefficient between the ball screw and the movable carriage.